1. Field of the Invention
The present invention relates to a method for recovering carboxylic acids from dilute aqueous streams via liquid-liquid extraction with a solvent comprising pressurized liquefied propylene and/or propane. More particularly, the present invention relates to a method for recovering acrylic acid from dilute acid water streams in processes for the manufacture of acrolein or acrylic acid.
2. Discussion of the Prior Art
Both acrolein and acrylic Acid (AA) are conventionally produced by gas-phase catalytic oxidation of propylene. There are also reports in the patent and open literature that with suitable catalysts, propane can be used as a feedstock in lieu of propylene.
In acrolein manufacture, the reaction is typically carried out in a single-stage reactor, optimized to selectively oxidize propylene or propane to acrolein, with a minimum of byproducts. However, some over-oxidation occurs resulting in the production of AA as well. In AA manufacture, the reaction is typically rallied out in two stages, oxidizing propylene or propane to acrolein in the first stage (as in acrolein manufacture), and then further oxidizing the acrolein to AA in the second stage.
In both acrolein and AA manufacture, AA is first separated from the gas phase reactor effluent by absorption into water, resulting in a dilute aqueous AA stream that also contains water-soluble, medium- and higher-boiling reaction byproduct impurities such as acetone, allyl alcohol, acetic acid, propionic acid, and maleic acid.
In AA manufacture, the aqueous AA stream leaving the absorber typically contains less than 40-65% AA. This crude aqueous AA stream is sent to a purification system which typically involves a series of energy-intensive distillation columns. Because of the relatively high water content, and the fact that AA forms azeotropes with water and other reaction impurities, the AA purification system is complex and energy intensive.
In acrolein manufacture, the aqueous AA stream leaving the absorber typically contains less than 10% AA; at stand-alone manufacturing sites, it is handled as a waste. Although dilute in AA, the concentrations are sufficiently high to make it impractical to treat the wastewater by relatively low-cost means such as biological treatment or wet-air oxidation. Thus, the AA wastewater is typically sent to an incinerator. Operating the incinerator requires a large amount of fuel owing to the large amount of water present, relative to the AA. Thus, handling and incineration of the dilute AA stream represents a significant operating expense in the manufacture of acrolein. While in principle the aqueous AA could be transported to an off-site AA manufacturing facility for recovery of the AA, the dilute AA concentration makes transportation costs prohibitive.
In the case of acrolein manufacture, it would be desirable to separate and recover AA from the dilute wastewater stream, yielding an AA concentrate, and an AA-depleted wastewater. The AA concentrated could be transported economically to an off-site AA manufacturing facility, for purification of the AA into a commercially valuable product. The AA-depleted wastewater may have a sufficiently low concentration of residual organic compounds, so that it can be feasibly treated by less expensive means than incineration, e.g. biological wastewater treatment or wet-air oxidation.
In case of AA manufacture, it would be desirable to separate the AA from the water in crude AA exiting the absorber. Reducing the water loading in the crude AA sent to purification reduces the energy requirements and costs in the distillation train.
The prior art describes various extraction-based processes for separating carboxylic acids, especially AA, from aqueous streams. However, most of these involve solvents are liquid at ambient temperatures, and often involve solvents whose boiling points are higher than that of water or water-AA mixtures. This makes it cumbersome to recover the solvent in at sufficiently high purity to allow recycle to extraction without causing a build-up of undesirable impurities. The use of these alternative solvents has the further disadvantage of requiring the handling of additional materials in the manufacture of acrolein or AA.
Alternative solvents that are more volatile than AA do not have a high enough relative volatility to afford a clean separation in a single-stage flash from either AA or some of the other impurities (propylene oxidation reaction byproducts) that co-extract with the AA. This requires that a more complex purification process (e.g. fractional distillation) be used to purify the solvent for recycle. Without purification, the solvent would accumulate impurities or AA, limiting the efficiency of the extraction step.
In order to avoid taking the carboxylic acids as bottom streams, the alternative is to use solvents which are less volatile, i.e. high-boiling, than the carboxylic acids, as the carboxylic acids are boiled overhead, and the solvent generally is taken as the bottoms streams. This can lead to increased polymerization or fouling due to the relatively high temperatures required to boil-up AA (even under vacuum), as well as the propensity of uninhibited AA vapors to polymerize when re-condensing. The fouling tendency can be mitigated by reducing the pressure of the distillations and for the addition of polymerization inhibitors to the distillation system. This is well known in the art.
Many AA extraction solvents described in the prior art are somewhat polar materials, rather than simple non-polar hydrocarbons. Consequently, the AA solutions in the polar solvents tend to form azeotropes, which further complicates the downstream purification of AA.
It is desirable to be able to extract AA without using additional chemicals (e.g. solvents) that are not otherwise required in the manufacturing process; i.e. to use only materials in the synthesis process as the extraction agent, as this avoids the logistics, costs, hazards, permitting, and material handling issues associated with introducing a new chemical into a manufacturing facility.
Prior art related to Acrylic Acid separation using solvent extraction includes the following:
U.S. Pat. No. 6,995,282 discloses the use of at least one heavy hydrophobic absorption solvent having a boiling point at atmospheric pressure of greater than 200° C. carboxylic acids from aqueous solutions.
U.S. Pat. No. 3,868,417 discloses carboxylic esters of melting point less than 30° C. and boiling point at normal pressure greater than 160° C. at elevated temperature and a pressure of 0.5 to 5 bars such as methyl, ethyl, n-butyl, iso-octyl-2-ethylhexyl and/or octyl esters of oleic acid, adipic acid and/or phthalic acid carboxylic acids from aqueous solutions.
U.S. Pat. No. 3,868,175 discloses the use of a dual solvent consisting of the first component capable of forming an azeotropic mixture with acrylic acid, acetic acid and water and the second component having a lower boiling point than that of acetic acid. The first component used is in an azeotropic relation with acrylic acid and acetic acid, and includes, for example, ethyl benzene, o-xylene, m-xylene, p-xylene, and octane. The second component has a boiling point lower than that of acetic acid, and includes, for example, methylethylketone, methyl acetate, and ethyl acetate for recovering carboxylic acids such as acrylic acid from aqueous solutions.
U.S. Pat. No. 6,737,546 discloses of the use of an immiscible solvent comprising propyl acetate and a cyclohexane and an integrated sequence of distillations and phase separations to separate the desired product or products and recover for recycle organic components of the extraction solvent.
U.S. Pat. No. 5,399,751 discloses the use a solvent consisting essentially of mixed trialkylphosphine oxides for recovering carboxylic acids from aqueous solutions.
Canadian Patent Application CA 2 282 492, discloses the use of materials that can be converted into (meth)acrylic acid as the extracting agents for extracting methacylic or acrylic acids from aqueous solutions.